Experimental analysis of alkali-brine-alcohol phase behavior with high acid number crude oil

A potential enhanced oil recovery (EOR) technique is to inject alkali into a reservoir with a high total-acid-number (TAN) crude to generate soap in situ and reduce interfacial tension (IFT). The method may be cost-effective if the IFT can be lowered enough to cause significant mobilization of trapped oil, while also avoiding formation of gel/viscous microemulsions. This paper investigates the potential field application of injecting alkali to generate in situ soap and favorable phase behavior for a high TAN oil. Oil analysis results show that the acids in the crude are a complex mixture of various polar species and not mainly carboxylic acids. Phase behavior experiments show that the system does not undergo typical Winsor microemulsion behavior. Therefore, traditional microemulsion models can cause unreliable recovery estimates. The mixing of alkali and crude/brine can generate water-in-oil macroemulsions that are highly viscous instead of forming the classical Winsor types. Good core flood recovery cannot be explained by the formation of a Winsor microemulsion phase, as is expected in surfactant EOR, mainly because these macroemulsions form before such idealized phase behavior can occur. A substantial decrease in interfacial tension is observed without the formation of a viscous phase in a narrow window of alkali concentration. Corefloods with polymer perform well in this concentration range, although incremental recovery could be overestimated some owing to increased water solubility in oil within these macroemulsions. The viscous phase behavior at large alkali concentrations is likely explained by the formation of salt-crude complexes, created by acids from the crude oil under the alkali environment. These hydrophobic molecules tend to agglomerate at the oil-water interface. Together with polar components from the crude oil, they can organize into a highly viscous network and stabilize water droplets in the oleic phase. Oil-soluble alcohol was added to counter those two phenomena at large concentrations, but typical Winsor phase behavior was still not observed. A physicochemical model is proposed to explain the salt-crude complex formation at the oil-water interface that inhibits classical Winsor behavior.